1. Field of the Invention
The invention relates to ceramic frits having excellent whiteness and covering power and also having a very high gloss and very little wrinkling after high-speed ceramic baking between 980.degree. C. and 1150.degree. C., the frits being free from the toxic element lead.
2. Description of the Related Art
Numerous ceramic frits for stoving on ceramic substrates are known from the literature.
For example P. Vincenzini et al. in Ceramurgia 5, No. 1, page 3 (1975) describe TiO.sub.2 frits having the following composition:
______________________________________ SiO.sub.2 51-61 wt. % Al.sub.2 O.sub.3 5-8 wt. % B.sub.2 O.sub.3 9-14 wt. % TiO.sub.2 4.5-7.5 wt. % PbO 0-7 wt. % CaO 4-7 wt. % Na.sub.2 O 2.5-5 wt. % K.sub.2 O 0-1 wt. % ______________________________________
These frits, however, are unsuitable for high-speed baking on porous unbaked bodies. The edge softening point is too low, i.e. the frit sinters before the body has completely degassed. This results in faulty, blistery glaze coatings. The frits are also unsuitable for high-speed baking on baked bodies. During baking, they give faulty surfaces owing to formation of pits, dents or pin-holes. These lead-containing frits are also very hazardous for toxic and ecological reasons.
British patents 808 041 describes frits having the following composition:
______________________________________ SiO.sub.2 50-65 mol. % Al.sub.2 O.sub.3 4-12 mol. % TiO.sub.2 6-18 mol. % ZrO.sub.2 0-6 mol. % Na.sub.2 O or Na.sub.2 O + K.sub.2 O 10-18 mol. % Na.sub.2 O .gtoreq.5 mol. % MgO + CaO + SrO 5-15 mol. % ______________________________________
These substances likewise are unsuitable for high-speed baking and give yellowish glazes. The high TiO.sub.2 content also increases the expense of the frits.
K. H. Styhr and M. D. Beals in Ceramic Bulletin 37, No. 11, page 480 (1958) describe frits having the composition:
______________________________________ SiO.sub.2 55-65 wt. % B.sub.2 O.sub.3 6-12 wt. % Al.sub.2 O.sub.3 6-14 wt. % TiO.sub.2 5-20 wt. % Na.sub.2 O 4-10 wt. % K.sub.2 O 0-8 wt. % Li.sub.2 O 0-8 wt. % PbO 0-6 wt. % ZnO 0-10 wt. % MgO 0-8 wt. % CaO 0-4 wt. % SrO 0-4 wt. % BaO 0-6 wt. % ______________________________________
These also give glazes with a faulty blistery surface after ceramic high-speed baking.